Modern radiation therapy techniques include the use of Intensity Modulated Radiotherapy (“IMRT”), typically by means of an external radiation treatment system, such as a linear accelerator, equipped with a multi-leaf collimator (“MLC”). Use of multi-leaf collimators in general, and an IMRT field in particular, allows the radiologist to treat a patient from a given direction of incidence to the target while varying the shape and dose of the radiation beam, thereby providing greatly enhanced ability to deliver radiation to a target within a treatment volume while avoiding excess irradiation of nearby healthy tissue. However, the greater freedom IMRT and other complex radiotherapy techniques, such as volumetric modulated arc therapy (VMAT, where the system gantry moves while radiation is delivered) and three-dimensional conformal radiotherapy (“3D conformal” or “3DCRT”), afford to radiologists has made the task of developing treatment plans more difficult. As used herein, the term radiotherapy should be broadly construed and is intended to include various techniques used to irradiate a patient, including use of photons (such as high energy x-rays and gamma rays), particles (such as electron and proton beams), and radiosurgical techniques. While modern linear accelerators use MLCs, other methods of providing conformal radiation to a target volume are known and are within the scope of the present invention.
Several techniques have been developed to create radiation treatment plans for IMRT or conformal radiation therapy. Generally, these techniques are directed to solving the “inverse” problem of determining the optimal combination of angles, radiation doses and MLC leaf movements to deliver the desired total radiation dose to the target, or possibly multiple targets, while minimizing irradiation of healthy tissue. This inverse problem is even more complex for developing arc therapy plans where the gantry is in motion while irradiating the target volume. Heretofore, radiation oncologists or other medical professionals, such as medical physicists and dosimetrists, have used one of the available algorithms to develop and optimize a radiation treatment plan.
When treating multiple targets concurrently using volumetric modulated arc therapy (VMAT), a conflict may arise when there are two or more targets separated from each other under a single pair of MLC leaves for a certain portion of a VMAT arc. If the aperture formed by the MLC leaf pair is wide enough so as to expose all participating targets, normal tissue located in a region between the participating targets may receive unwanted radiation doses. Therefore, it is desirable to generate MLC sequences that can minimize radiation exposure to normal tissue located in the region between the participating targets.